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As mentioned before, PCT considers only chemically ordered networks which, according to Zachariasen [2], are TD networks of structural units made up of corner‐sharing rigid polyhedra. In such networks, it is convenient to treat the shared corners of the polyhedra as the vertices and the polyhedral structural units as links. The rigidity of a network then arises from the rigidity of the structural units as well as from the vertex‐connectivity condition (i.e. the fact that all corners of polyhedral units are shared among a certain number of units). These constraints resulting from the rigidity of structural units and connectivity are termed polyhedral constraints.

The deformation of a TD network is determined by the type of polyhedral structural units and by the vertex‐connectivity (C) that specifies the number of polyhedral units sharing a vertex. The vertex‐connectivity is related to (see Eq. 1) but is different from the average coordination number r.